1. Field of the Invention
The present invention relates to a liquid sealed vibration isolating device for use in an engine mount for a motor vehicle, which is provided with a resonance orifice passage to isolate vibrations by liquid column resonance of the resonance orifice passage, and more particularly, to the device capable of widening a range of a resonance frequency.
2. Description of the Related Art
FIG. 8 is a cross sectional view showing a conventional liquid sealed engine mount and FIG. 9 is an enlarged cross sectional view showing a diaphragm section. Referring to these drawings, a damping orifice passage 107 and an idle orifice passage 115 are provided in a partition member 106 to communicate between a primary liquid chamber 105 enclosed by an insulator 104 and a secondary liquid chamber 108 enclosed by a diaphragm 109. The idle orifice passage 115 is opened and closed by a valve portion 117 provided in a portion of the diaphragm 109. The valve portion 117 is formed considerably thicker than a surrounding portion 131 of the diaphragm 109 which surrounds the valve portion 117. The surrounding portion 131 is obliquely connected to an outer peripheral portion of the valve portion 117.
Patent reference 1: Japanese patent laid open publication No. 2004-324826.
By the way, the diaphragm 109 in the above structure has hardly any spring and produces no membrane resonance in a range effective in isolating the vibration, so that a membrane characteristic of the diaphragm 109 itself does not contribute to a decrease in dynamic spring constant. In particular, if the valve portion 117 is capable of being elastically deformed when the valve portion 117 is closed, it can contribute to the decrease in dynamic spring constant, but it is too thick to expect such contribution. Further, when the valve portion 117 is opened, the diaphragm 109 is easily deformed entirely and the valve portion 117 is not elastically deformed whether the central valve portion 117 has high rigidity. Even if the valve portion 117 starts deformation, the surrounding portion 131 finishes deformation first since it is easy to deform, so as not to cause the valve portion 117 to be elastically deformed. Thus, the diaphragm 109 as a whole does not produce the membrane resonance in the effective range in vibration isolation.
Accordingly, the present invention has its object to provide a liquid sealed vibration isolating device in which the diaphragm membrane-resonates in a desirable frequency range of the effective range in vibration isolation so as to contribute to the decrease in dynamic spring constant.